This application claims the priority benefit of Japanese application serial no. 2001-352986, filed on Nov. 19, 2001.
The present invention relates to a capillary for optical fiber, a ferrule for optical connector, and an optical-fiber-fixed capillary in which an optical fiber is inserted and fixed into an inner hole of a capillary for optical fiber, which are used for manufacturing optical devices.
Recent rapid development of optical communication networks has raised the need for large quantities of high performance, inexpensive optical devices. For a plug-type optical device or a receptacle-type optical device with a built-in optical fiber, in particular, a cylindrical optical-fiber-fixed capillary, called optical fiber stub, formed by inserting an optical fiber into an precise capillary to be fixed with an adhesive, and an optical connector plug are employed.
For example, as shown in FIG. 12, when one end face of an optical-fiber-fixed capillary is ground into a convex-sphere and the other end face is ground (or polished) into an inclined surface, the capillary comes to work as an optical fiber stub 6. The optical fiber stub 6 is used for receiving an optical signal emitted from a laser diode 1 and focused by a lens 2, transmitting the signal to an optical fiber 5 in a ferrule 4 for optical connector of an optical connector plug 3, or for making the signal emitted from the optical fiber 5 in an inner hole 4a of the ferrule 4 for optical connector focus on a photodiode and the like which is not graphically represented.
An end face 7b of the optical fiber stub 6 is ground so that the reflection axis of the optical signal makes an angle of several degrees against the incident axis of the optical signal. This arrangement is made to prevent such a phenomenon from occurring at the end face 7b of the capillary 7 for optical fiber on the side of the laser diode 1 (or photodiode) that a reflected light comes into the laser diode 1 and becomes a noise. An end face 7c opposite to the end face 7b is ground to form a convex-sphere having a center on an end face 8a of an optical fiber 8 so that the optical fiber 8 is abutted on the optical connector plug 3 to be able to make PC (Physical Contact) connection.
As shown in FIG. 12, the connection between the optical connector plug 3 and the optical fiber stub 6 is made via PC connection, for which the end face 8a of the optical fiber 8 of the optical fiber stub 6 is abutted on a convex-spherical end face 5a of an optical fiber 5 of the optical connector plug 3 in a split sleeve 9.
Such an optical signal connection as described above is made typically by connecting optical connectors used for high capacity optical communication. For connecting optical connectors, as shown in FIG. 13, a ferrule 11 for optical connector is employed to form an optical connector plug 10, the ferrule 11 formed by processing an precise capillary, which has an inner hole 11 a with an inner diameter for allowing an optical fiber 12 to be inserted therein, into a prescribed shape. The optical fiber 12 is inserted into the inner hole 11a to be glued thereto with an adhesive and one end face of the ferrule 11 is machined into a convex-sphere. The connection between these optical plugs 10 are made via PC connection, in which respective end face 12a of the optical fiber 12 abut each other in a split sleeve 13. A connection loss resulted from the PC connection mainly caused by an axial dislocation between the optical fibers 12 occurring at the connecting part. The axial dislocation occurs as the result from accumulated factors of the outer diameter difference between used ferrules 11 for optical connector, the circularity (or roundness) of the inner hole 11a at the end face and the cylindricity of an outer periphery 11b, the concentricity between the inner hole 11a of the ferrule 11 for optical connector and the outer periphery 11b, and the eccentricity of the optical fiber 12 in the inner hole 12 at the end face. Out of these factors, the concentricity between the inner hole 11a of the ferrule 11 for optical connector at the end face and the outer periphery 11b, and the eccentricity of the optical fiber 12 in the inner hole 11a at the end face are the major factors relating to the axial dislocation between the optical fibers to be abutted. With this reason, the concentricity of the ferrule 11 for optical connector is required to be 1.4 xcexcm or less for an application to communication using a single mode optical fiber. Also, a ferrule having the inner hole 11a larger than the outer diameter of the optical fiber 12 to be inserted by 0 to 1 xcexcm is employed for such a communication. The relation between the axial dislocation d of the optical fiber 12 and a connection loss (measuring unit: dB) is represented generally by the following equation (1) when the core diameter of the optical fiber is symbolized by w.
Loss=4.34(d/(w/2))2xe2x80x83xe2x80x83(1) 
Here, a consideration is made on the cause of connection loss by focusing on the concentricity between the inner hole 11a and the outer periphery 11b, and the eccentricity of the optical fiber 12 in the inner hole 11a. A case to be considered first is to use the ferrule 11 for optical connector, whose concentricity is 1.4 xcexcm or less and the inner hole 11a has an inner diameter larger than the outer diameter of the optical fiber 12 by 1 xcexcm. When the optical fiber becomes eccentric in the inner hole 11a by 0.5 xcexcm in one ferrule on the assumption that the other ferrule to be abutted is subjected to the same eccentricity, the worst axial dislocation to be estimated is 2.4 xcexcm, which is the dislocation value bringing a connection loss of 1.0 dB in maximum according to the above equation (1). Given this conclusion, the ferrule 11 for optical fiber must be revolved in a core adjusting operation in order to achieve a connection loss of 0.5 dB or less, which is specified as the standard value of an communication optical connector employing a quartz single mode optical fiber (JIS-C-5962). Actual core adjusting operation, however, tends to be extraordinary cumbersome. For example, the concentricity between the inner hole 11a and the outer periphery 11b is measured first at the end face of the ferrule 11 for optical connector, and an eccentric direction is marked on the side of a flange member 14 with 90 degree pitch (xc2xc revolution pitch), then assembling is made by aligning the eccentric direction of respective ferrule 11 for optical connection to be abutted. Or, in another example, while light is actually transmitted through the optical fiber and the intensity of the light is monitored with a power meter and the like, the ferrule 11 for optical fiber is revolved 90 degree at a time and the position for least connection loss is determined for assembling.
Meanwhile, the optical device represented by FIG. 12 is also subjected to the same axial dislocation described above. In this case, when the optical connector plug 3 and the optical fiber stub 6 is connected or an aligning position is determined between the optical axis of an optical signal focused by the lens 2 (or an emitted optical signal) and the center of the optical fiber 8 in the inner hole 7a of the capillary 7 for optical fiber used for the optical fiber stub 6, the end face 7b of the capillary 7 for optical fiber used for optical fiber stub 6 is ground (or polishing) to make an angle of several degrees and is fixed without a consideration for the eccentric direction of the optical fiber in the inner hole 7a, so that the core adjusting operation becomes structurally impossible. Therefore, as the optical connector plug and the optical fiber stub to be abutted have respective axial dislocation, the axial dislocation caused by the eccentricity of the optical fiber 8 in the inner hole 7a can not be reduced, so that the connection loss of optical signal increases.
For the mass production of the ferrule 11 for optical connector, as shown FIG. 13, with a severe dimensional accuracy that the tolerance of concentricity is 1.4 xcexcm or less and the tolerance of inner diameter against the diameter of the optical fiber 12 isxe2x80x940 xcexcm/+1.0 xcexcm, for example, when the ferrule 11 for optical connector is made of commonly employed zirconia ceramics, both the inner and outer surface of the ferrule 11 need to be ground so as to have a surface roughness with Ra value of less than 0.1 xcexcm, because a sintered form of zirconia ceramics is processed to be used as the material for the ferrule.
It is known that when such a zirconia ceramic ferrule 11 for optical connector has the inner hole 11a having the surface roughness with Ra value of less than 0.1 and the optical fiber 12 is inserted and glued in the inner hole 11a, the optical fiber 12 tends to be decentered toward the side wall of the inner hole 11a and is difficult to be adjusted to the center of the inner hole 11a. The inventors of this invention have also confirmed the above fact. Therefore, the cumbersome core adjustment operation mentioned above has been imperative for the conventional zirconia ceramic ferrule 11 for optical connector, which has a small surface roughness, in order to achieve the specified connection loss value of 0.5 dB or less, which is registered as the standard for the quartz single mode optical fiber communication optical connector (JIS-C-5962). When connection between the optical fiber stub 6 and the optical connector plug 3 is made, as shown in FIG. 12, the same eccentric phenomenon occurs. In this case, the core adjusting operation can not be carried out due to the structural characteristics of the optical fiber stub 6, which makes it impossible to reduce the axial dislocation caused by the eccentricity of the optical fiber 8 in the inner hole 7a when the conventional zirconia ceramic capillary 7 for optical fiber is employed, so that the problem of increasing light signal loss remains.
Also, for optical devices of high capacity and/or high reliability applied to optical communication, a further lower connection loss, lower than the specified value for optical connector connection loss of 0.5 dB, may be required. To meet such a requirement, there is no other way but to set the stricter tolerance of the inner diameter 11a or of the concentricity of the ferrule 11 for optical connector shown in FIG. 13. With this reason, a number of ferrules 11 for optical connector are required to be manufactured so that a one with a good dimensional accuracy can be selected. As a result, the manufacturing cost of ferrules becomes higher. The optical fiber stub 6 formed by processing an optical-fiber-fixed capillary, as shown in FIG. 12, has the PC connection part with the structure equal to that of the ferrule for optical connector, making it impossible to carry out the core adjusting operation when a low connector connection loss equal to the above is required, which leads also to a stricter accuracy requirement to raise the problem of a higher production cost.
The object of the invention is to provide a capillary for optical fiber, which enables a connection loss lower than that of a conventional level, and a ferrule for optical connector and an optical-fiber-fixed capillary both using the capillary for optical fiber.
To achieve the above object, the inventors have accomplished the following invention. A capillary for optical fiber according to the present invention comprises an inner hole with an inner diameter allowing an optical fiber to be inserted and retained therein, wherein the Ra value of the surface roughness of the inner hole is 0.1 xcexcm to 0.5 xcexcm.
When the surface roughness Ra value of the inner hole of a capillary for optical fiber is less than 0.1 xcexcm, an inserted optical fiber tends to approach the side wall of the inner hole to become eccentric, so that a core adjusting effect can not be expected. On the other hand, when the surface roughness Ra value of the inner hole exceeds 0.5 xcexcm, Ry value defined as the maximum roughness is also expected to be considerably large. In such a case, the center of the maximum inscribed cylinder of the inner hole is decenterd in many cases against the center of the circle determined by the average line of the surface roughness of the inner hole, so that the original concentricity of the inner hole virtually deteriorates, or the surface of the inserted optical fiber may suffer scratches, raising a fear of decreasing strength of the optical fiber. It is essential for the capillary for optical fiber according to the present invention that the surface roughness Ra value of the inner hole is to be 0.1 xcexcm to 0.5 xcexcm.
For example, when an optical connector plug or an optical fiber stub is comprised of a capillary for optical fiber made of zirconia ceramics, the inner hole of the capillary is ground precisely to keep its surface roughness Ra value less than 0.1 xcexcm and the strict dimensional accuracy (the inner diameter tolerance ofxe2x80x940 xcexcm/+1.0 xcexcm, concentricity tolerance of 1.4 xcexcm or less) is achieved, so that the connection loss requirement for optical connector is met. However, when the surface roughness Ra value exceeds 0.5 xcexcm, satisfying the above dimensional accuracy is difficult and mass production of optical connector plugs or optical fiber stubs becomes almost impossible.
Likewise, when an optical connector plug or optical fiber stub is comprised of a crystallized glass capillary for optical fiber manufactured by drawing forming, the surface roughness Ra value of over 0.5 xcexcm makes it difficult to satisfy the strict dimensional accuracy required for capillaries for an optical fiber stub, so that mass production of inexpensive optical fiber stubs becomes also difficult.
The capillary for optical fiber according to the present invention has Ry value of the inner hole surface roughness of 4.0 xcexcm or less.
There is a possibility, when the surface roughness Ry value of a capillary for optical fiber exceeds 4.0 xcexcm, that the center of the maximum inscribed cylinder of the inner hole may be decentered against the center of the circle determined by the average line of the surface roughness of the inner hole. If this happens, the original concentricity of the inner hole virtually deteriorates and projected sharp peaks of the surface roughness of the inner hole may damage the surface of an inserted optical fiber. Therefore, it is essential for the capillary for optical fiber of the present invention that Ry value of the inner hole surface roughness is 0.4 xcexcm or less.
The capillary for optical fiber according to the present invention has the inner hole in which the difference between the average line and the peak line of the surface roughness is 2.0 xcexcm or less.
When the difference between the average line and the peak line of the surface roughness of the inner hole of a capillary for optical fiber, a specific value of the difference being defined in JIS-B-0601, exceeds 2.0 xcexcm, there is a possibility that the center of the maximum inscribed cylinder of the inner hole may be decentered against the center of the circle determined by the average line of the surface roughness of the inner hole. If this happens, the original concentricity of the inner hole virtually deteriorates and projected sharp peaks of the surface roughness of the inner hole may damage the surface of an inserted optical fiber. When the capillary for optical fiber of the present invention is employed for comprising an optical-fiber-fixed capillary, such as an optical connector plug or an optical fiber stub, it is essential that the difference between the average line and the peak line of the surface roughness is 2.0 xcexcm or less, even if the peaks and valleys of the surface roughness are not symmetrically with each other with respect to the average line of the surface roughness, in order to obtain the core adjusting effect for the optical fiber in the inner hole.
The capillary for optical fiber according to the present invention is made of crystallized glass formed by precipitating crystals in amorphous glass, wherein crystallized grains are precipitated on the surface of the inner hole.
As the crystal precipitated in amorphous glass in the present invention, a crystal grain having diameter of 0.1 xcexcm to 1.0 xcexcm is applicable to meet the condition that the Ra value of the surface roughness of the inner hole of the capillary for optical fiber is 0.1, to 0.5 xcexcm. For example, xcex2-spodumene crystal and the like are suitable.
The capillary for optical fiber according to the present invention has the inner hole, of which the surface has a prescribed surface roughness formed by a mechanical process mean.
As the mechanical process mean for providing the prescribed surface roughness on the inner hole surface of the capillary for optical fiber of the present invention, for example, an internal surface grinding or polishing, using a wire or abrasive grain, is generally performed to obtain a good dimensional accuracy upon manufacturing a capillary for optical fiber made of zirconia ceramics and the like. In this case, the surface roughness can be intentionally controlled within a prescribed range by using an internal grinding (or polishing) machine and the like before executing the finish grinding (or polishing). Such a process mean makes it possible to form an inner hole having the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm, an inner hole having the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and the surface roughness Ry value of 4.0 xcexcm or less, an inner hole having the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and the difference between the average line and the peak line of 2.0 xcexcm or less, and an inner hole having both the above Ra value and Ry value and the difference xcex4 between the average line and the peak line of 2.0 xcexcm or less.
The description that the inner hole of the capillary for optical fiber has the inner diameter allowing an optical fiber to be inserted therein means that a pin gage having a diameter equal to that of the optical fiber can be inserted into the inner hole. The description that the inner diameter is larger than the optical fiber by 1 xcexcm means that a pin gage having a diameter larger than the optical fiber by 1 xcexcm can be inserted into the inner hole and a pin gage having a diameter surpassing such a size can not be inserted even if the surpassed size is a slight one. No gap between the inner hole and the optical fiber makes it impossible for the optical fiber to be inserted, and more than 2.0 xcexcm gap makes an adhesive injected on the outer periphery of the optical fiber move along the periphery to gather in, increasing the eccentricity of the optical fiber in the inner hole.
The ferrule for optical connector according to the present invention is that the capillary for optical fiber as described above is formed with a chamfered part on one end thereof for guiding the ferrule into a sleeve, and a flared part opened to the other end thereof for guiding the optical fiber into the inner hole.
The ferrule for optical connector of the present invention is applicable when it has an outer diameter, an inner diameter, concentricity, and cylindricity, all of which allow the ferrule to be connected to an optical connector plug with a prescribed connection loss. The chamfered part for guiding the ferrule into the sleeve is applicable when the chamfered part has a dimension and shape equal to that of a known ferrule for optical fiber. As the flared part, it is desirable that it has a shape allowing the optical fiber to be easily inserted into the inner hole and a dimension preventing wide fluctuation of the connection loss caused by an ambient temperature change and the like after fixing the optical fiber. Further, a preliminary convex-spherical grinding (or polishing) for PC connection may be carried out, with putting the center of grinding (or polishing) on the end face of the inner hole into which the optical fiber is inserted, so that the ferrule can be connected to an optical connector plug via PC connection.
The optical-fiber-fixed capillary according to the present invention comprises the capillary for optical fiber as described above and an optical fiber inserted and fixed into the inner hole of the capillary for optical fiber.
The optical-fiber-fixed capillary of the present invention is provided in the form of an optical connector plug, an optical fiber stub, and a lengthy optical-fiber-fixed preliminary material, etc. These optical devices are applicable if they are the one allowing an optical fiber to be positioned precisely in the center of a capillary for optical fiber. When the optical plugs or optical fiber stubs and the like are abutted and connected in a precise sleeve, it is desirable that a chamfered part is formed at least one end face of the capillary for optical fiber or that a convex-spherical grinding (or polishing) is performed on the end face, with putting the center of grinding (or polishing) on the end of optical fiber, so as to make PC connection to an optical plug. The end face of the capillary for optical fiber is ground in such a way that the end face makes an angle of several degrees against the incident axis of a light signal in order to prevent a reflected light from coming into a laser diode to make noises. As an adhesive for gluing the optical fiber into the inner hole of the capillary for optical fiber, an epoxy-type one with proven effectiveness, an ultraviolet-curing-type one with excellent operability, and the like are appropriate according to the purpose of application.
Since the capillary for optical fiber according to the present invention has the inner hole having the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm, due to the affection of the peaks higher than the average line and dispersed on the inner hole surface, the adhesive, including the epoxy-type one with proper viscosity, spreads uniformly in the circumferential direction to be retained on the surface of the inner hole in a stable manner, holding the optical fiber in the central position of the inner hole.
Also, the inner hole of the capillary for optical fiber of the present invention has the surface roughness Ry value of 4.0 xcexcm or less, which prevents the center of the maximum inscribed cylinder of the inner hole from decentered against the center of the circle determined by the average line of the surface roughness of the inner hole. This fact ensures that a high circularity of the inner hole and the concentricity between the inner hole and the outer periphery of the capillary are retained so that a desired core adjusting effect is obtained and the surface of inserted optical fiber is not damaged.
Further, in the capillary for optical fiber of the present invention, the difference between the average line and the peak line of the surface roughness in the inner hole is 2.0 xcexcm or less. Because of this fact, the center of the maximum inscribed cylinder of the inner hole is not decentered against the center of the circle determined by the average line of the surface roughness of the inner hole, even if the peaks and valleys of the surface roughness are not symmetrically with each other with respect to the average line of the surface roughness, so that the high circularity of the inner hole and the concentricity between the inner hole and the outer periphery of the capillary is secured and the surface of inserted optical fiber is not damaged. Thus, the desired core adjustment effect is obtained.
The capillary for optical fiber of the present invention is made of crystallized glass formed by precipitating crystals in amorphous glass. The crystallized glass is subjected to a heat treatment under a proper condition to precipitate crystallized grains on the surface of the inner hole, where the grains form the peaks. This process enables easy, effective manufacturing of the capillary having the inner hole with the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm, the capillary having the inner hole with the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and Ry value of 4.0 xcexcm or less, the capillary having the inner hole with the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and the difference between the average line and the peak line of 2.0 xcexcm or less, and the capillary having the inner hole with the above surface roughness Ra and Ry values and the difference between the average line and the peak line of 2.0 xcexcm or less.
In the capillary for optical fiber of the present invention, the inner hole has a prescribed surface roughness formed by a mechanical process mean. Therefore, the inner hole with surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm, the capillary having the inner hole with the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and Ry value of 4.0 xcexcm or less, the capillary having the inner hole with the surface roughness Ra value of 0.1 xcexcm to 0.5 xcexcm and the difference between the average line and the peak line of 2.0 xcexcm or less, and the capillary having the inner hole with the above surface roughness Ra and Ry values and the difference between the average line and the peak line of 2.0 xcexcm or less, are manufactured under a room temperature by using the internal grinding (or polishing) machine and the like.
The ferrule for optical connector of the present invention is that the capillary for optical fiber as described above is formed with the chamfered part on one end thereof for guiding the ferrule into the sleeve and the flared part opened to the other end for guiding an optical fiber into the inner hole. Therefore, the optical fiber can be held precisely in the center of the inner hole of the ferrule for optical connector by the core adjusting effect produced by the above surface roughness of the inner hole of the invented capillary for optical fiber.
The optical-fiber-fixed capillary of the present invention comprises the capillary for optical fiber as described above and the optical fiber inserted and fixed into the inner hole of the capillary for optical fiber. Therefore, there is provided the optical-fiber-fixed capillary, in which the optical fiber is retained in the center of the inner hole by the core adjusting effect produced by the surface roughness of the inner hole.